Afghanistan rebuffed an American demand relayed today by Pakistan that the Taliban government immediately and unconditionally surrender Osama bin Laden to head off a military showdown, but Afghan leaders set a high-level meeting for Tuesday to make a final decision.

In a day of talks in the Afghan city of Kandahar, the hard-line Islamic clerics who lead the Taliban demanded ''convincing evidence'' that Mr. bin Laden was responsible for the attacks last week in New York and Washington, officials in Pakistan said. The Taliban also said they wanted the Organization of the Islamic Conference, a group of more than 50 Muslim countries, to formally demand Mr. bin Laden's handover.

Those conditions appeared unlikely to be acceptable to the United States, which has made clear its readiness to use military force to capture or kill Mr. bin Laden. But the Pakistani military officers who carried the American ultimatum decided to remain in Afghanistan overnight and resume their effort on Tuesday.

A spokesman for the Taliban said a council of about 600 clerics would assemble on Tuesday in Kabul, the capital, and make a binding decision on the fate of Mr. bin Laden. At least one of the Pakistani generals was invited to attend the clerics' meeting and present their case directly.

The clear implication of the Taliban's response seemed to be that they do know the whereabouts of Mr. bin Laden and would be in a position to hand him over if they decided to. But it also seemed possible that the Taliban were merely playing for time.

At the volatile border between Afghanistan and Pakistan, time appeared to be running short. There were at least two reports, one from a correspondent for the news agency Reuters quoting a Pakistani officer in the Khyber Pass, that the Taliban had moved military units, some with Russian-made Scud missiles, up to the border.

If the troop movements are confirmed, they would be consistent with threats from the Taliban that Afghanistan would attack Pakistan if its government supported American military action.

Afghanistan closed its airspace to all international airlines, warning that it would ''take action'' against any aircraft defying the order. About 110 flights a day normally fly over Afghanistan.

At the border, chaos mounted as Afghans streaming away from the threat of an American attack found border crossings sealed by Pakistani authorities at the request of the United States.

Scenes of anger and chaos erupted at Torkham, traditionally the busiest crossing point, at the place where the Khyber Pass climbs up into the mountains on the road leading to Kabul. The United Nations said that at least half the population of Kandahar, perhaps as many as 200,000 people, fled the city in recent days as the risks of an American strike mounted.

At Chaman, a frontier point near Kandahar, Pakistan state television reported 20,000 people had beaten the border closing and pressed into Pakistan within 24 hours. Pakistan is already awash in Afghan refugees who have fled drought and war and have sometimes found solace in the militant Islam of the Taliban, or of Mr. bin Laden himself.

After being briefed on the talks in telephone calls from Kandahar and Kabul, officials in Islamabad, Pakistan's capital, gave a gloomy assessment.

Pakistan has promised support for any American military operation, a decision that has risked unrest in Pakistan from militant Islamic groups opposed to this volatile nation of 140 million helping in an attack on a neighboring Muslim state.

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''Our dream, I cannot say expectation, is that somehow or other there will be a miracle,'' Pakistan's foreign minister, Abdul Sattar, said in an interview with the British Broadcasting Corporation a few hours after the Kandahar talks ended. ''We hope very much that the Taliban government will recognize the writing on the wall and take a decision in this light.''

Mr. Sattar said that the Taliban leaders had been told, ''Time is very short, patience has run out, there is no room for negotiation, it's time for action.''

In the meeting in Kandahar, Pakistan's military intelligence chief, Gen. Mahmood Ahmed, who was in Washington last week for crisis talks with the State Department and White House, told the Taliban leader, Mullah Mohammed Omar, that the Taliban faced an inevitable American military attack unless it handed over the Saudi-born Mr. bin Laden within a matter of days, the Pakistani officials said.

That meeting, lasting more than three hours, produced no convincing progress, the officials said, as was the case with two other meetings with the Taliban foreign minister, Abdul Salam Muttawakil, and the governor of Kandahar, Mullah Mohammed Hassan.

A further condition set by the Taliban for the handover of Mr. bin Laden was that any court trying Mr. bin Laden include at least one Muslim judge, officials said. This was an easing of an insistence by the Taliban, after earlier terrorist attacks, that the only court that would be allowed to try Mr. bin Laden would be an Islamic one.

Pakistani officials said the Taliban position had eased since Mullah Omar made a defiant radio speech on Friday pledging not to hand over Mr. bin Laden, but it has not eased enough, in Pakistan's estimation, to satisfy the United States.

They noted, in particular, Mullah Omar's agreement to consider a handover if the Taliban received evidence of Mr. bin Laden's culpability.

''Did you bring the evidence?'' the Taliban leader asked when he greeted the Pakistani delegation, an official said. The answer he was given, the official said, was a summary of the evidence already announced by Bush administration officials in Washington, but that did not appear to satisfy Taliban officials. Their demand was for documentary evidence, and for time to put the evidence before Islamic legal scholars, a stipulation that the Pakistani delegation viewed as a delaying tactic.

Officials in Islamabad were at pains to say that the meetings had not been hostile. This was hardly surprising, since Pakistan gave crucial backing to the Taliban in its rise to power after 1994. Mullah Omar, a village prayer leader and a one-eyed veteran of the decade-long Muslim guerrilla struggle against Soviet forces, has had a long-established relationship with several of the Pakistani officers who met him today.

The two top men in the delegation -- General Ahmed, and his deputy, Gen. Faiz Gilani, are the top figures in Pakistan's Inter Services Intelligence directorate, or I.S.I. responsible for channeling large amounts of military and financial aid to the Taliban. Until the attacks in New York and Washington, that support had been quietly tolerated by the United States, despite the bitter opposition to the repressive form of Islamic rule imposed by the Taliban.

For Mullah Omar, the talks represented a rare venture outside the secretive life he has chosen for himself in Kandahar, where he is said to spend his time in prayer and reading the Koran, has never been photographed or watched television, and shuns meetings with non-Muslims.

Last week, some of this occlusion seemed to be reflected in the radio address in which he denied that Mr. bin Laden could have had anything to do with the catastrophic hijackings and vowed not to hand him over. ''Osama could not have carried out this incident, and has not carried it out, and does not have opportunities to carry out such a process,'' he said.

Referring to the suicide pilots who carried out the attacks, he added: ''They were not afraid of America or anyone else. Those who sacrificed their lives didn't need Osama to be happy or not happy, or Osama to order them, or not.''